(Not applicable)
This invention relates generally to GPS-inertial navigation systems and more specifically to the compensation for changes in the GPS-receiver reference frequency resulting from environmental acceleration. The acronym xe2x80x9cGPSxe2x80x9d stands for the well-known Global Positioning System which provides the means for world-wide navigation using signals transmitted by a plurality of satellites.
The performance of GPS-inertial navigation systems in high-performance vehicles tends to be unacceptable in jamming environments. To accommodate the large Doppler shifts in frequency of the received GPS signals associated with high-performance vehicles, the filter bandwidths in the GPS receiver tracking loops must be increased, thereby making the GPS receiver more susceptible to jamming. Velocity aiding of GPS-receiver carrier-frequency tracking loops using inertial measurements permits the use of tracking-loop filters of lower order and reduced bandwidth with the accompanying advantage of reduced susceptibility to jamming signals. The degree to which the carrier-frequency tracking-loop filter bandwidth can be reduced is limited primarily by reference oscillator phase noise and by frequency shifting of the oscillator as a result of acceleration stress experienced by the crystal that serves as the oscillator frequency reference. There is a need for a means for compensating for the effects of acceleration in the GPS receiver.
The invention is a method and apparatus for compensating for acceleration in a GPS-inertial navigation system. A translated satellite signal is a satellite signal translated in the GPS receiver to an intermediate-frequency band by a sequence of M mixing operations, M being an integer greater than or equal to one. The m""th mixing operation consists of mixing the satellite signal with an m""th mixing signal having a frequency fm where fm can be approximated by a function Fm of one or more components A1, A2, and A3 of the acceleration and one or more parameters Pm1, Pm2, . . . , PmN(m) that are independent of acceleration.
The method consists of (a) obtaining values for one or more components of acceleration, (b) obtaining values for one or more parameters, (c) determining the value of one or more functions Fm, and (d) utilizing the data obtained in steps (a), (b), and (c) in compensating for the effects of acceleration in a GPS receiver.